1. Field of the Invention
The invention relates to an LED lighting device, in particular for the purpose of cabin lighting.
2. Description of Related Art
The market for lamps and lighting devices is subject to rapid technological development and is highly sensitive with regard to the manufacturing costs of such lighting devices. Especially light-emitting diodes (LEDs) define the forefront of current research efforts, for example in the field of general lighting, but for instance also in the automotive industry and in aviation.
LED lamps are increasingly being introduced wherever low energy consumption on the one hand and special design options on the other hand are desired.
For example, it is well known to accommodate a plurality of individual LEDs together in one lamp, either to increase light intensity, or to obtain for instance a 2-dimensional or, more generally, an expanded luminous surface area. In the field of general lighting, ceiling panels for open-plan offices can be mentioned here, which were previously implemented using fluorescent tubes, which now are more and more replaced by LEDs. In the automotive sector, by contrast, the design aspect is being increasingly exploited, which for instance allows to distribute LEDs along car body lines.
In aviation, for example in the area of cabin lighting in wide-body aircraft, it would also be desirable to replace the conventional light panels by modern LED lighting technology.
However, in aviation, the power supply of the cabin which sometimes is significantly fluctuating is a high hurdle. For example, it is possible that the auxiliary power unit (APU) of a wide-body aircraft provides a different voltage level than the ground power supply to which the aircraft is typically connected on the ground. Also, the voltage level of the ground power supply may vary from airport to airport. Furthermore, for instance the APU may supply a fluctuating voltage level. Finally, it also happens that over voltages arise that exceed the tolerated input voltage of the cabin lighting and may cause defects in the cabin lighting.
In prior art cabin lighting, voltage fluctuations are typically balanced in a manner so that the brightness of the cabin lighting fluctuates. That means, in case of an elevated voltage, power consumption of the cabin lighting will be higher. When the aircraft is connected to the ground power supply, brightness of the cabin lighting increases or drops, but also during the operation of the aircraft the brightness of cabin lighting may be subject to clearly visible fluctuations.
Therefore, a major technical challenge is to tolerate different voltage levels for a lighting device and at the same time ideally always provide the most consistent brightness possible with the lamp.
Another problem arises from the fact that LEDs are direct current (DC) devices which have to be connected to the typically existing AC voltage sources. This is particularly relevant with regard to the cost aspect, since as a matter of course many ways for obtaining DC voltage from AC voltage have already been known, for instance by using conventional power supply units, but further simplifications and/or cost savings are always sought for, for instance for the mass market. In particular, if stringent requirements are imposed on the power factor, conventional circuit complexity is significant.
Finally, it is moreover desirable to reduce the number of components which are necessary for connecting the lamp to the AC voltage source, for example in order to reduce maintenance costs, if the lamp is used in an application field where lamps are still maintained (e.g. in aviation).
Given this background, the applicant Schott AG has developed an LED lighting device which achieves the object of tolerating temporally varying voltage sources.
Moreover, the LED lighting device developed by the applicant is capable of achieving a particularly uniform brightness distribution. In particular, the developed LED lighting device is able to provide constant brightness even in case of a fluctuating amplitude of the AC voltage supply.
Finally, the developed LED lighting device is able to tolerate over voltages in a particularly simple manner so as to increase service life of the LED lighting device in an environment prone to over voltages.
Further objects will become apparent from the following description and the particular advantages obtained with specific embodiments.
The invention provides an LED lighting device for connection to an AC voltage source. The LED lighting device comprises at least a first and a second set of LEDs. Each set of LEDs comprises at least one LED assigned to the respective set of LEDs. Preferably, the LED sets are electrically arranged in series.
The first set of LEDs preferably comprises at least two LEDs assigned to the first set of LEDs and electrically arranged in series. In other words, the at least two LEDs are electrically connected in series; the first set of LEDs may for instance comprise three, four, five, six, or seven LEDs as well, which are electrically connected in series. In case of a plurality of LEDs per LED set, the same amperage passes through the LEDs of the first LED set so that all the LEDs of the first LED set essentially have the same brightness—apart from possible manufacturing variations of the LEDs. The LEDs of one set of LEDs, for example the first set of LEDs, are jointly enabled or disabled according to the invention, that means they form a unit in electrical terms.
The second set of LEDs also preferably comprises at least two LEDs assigned to the second set of LEDs and electrically arranged in series. The LEDs of the second set of LEDs can be regarded as electrically independent from the LEDs of the first set, so that although the LEDs assigned to the second set of LEDs are always considered together—in analogy to the LEDs of the first set of LEDs—but typically the LEDs of the first set are not enabled or disabled together with the LEDs of the second set.
In a preferred embodiment, further sets of LEDs are additionally provided, and each set of LEDs preferably comprises at least two LEDs, for example a third, a fourth, a fifth, a sixth and/or a seventh set of LEDs. To cite an example, an LED lighting device comprising 7 sets of LEDs and 6 LEDs per set of LEDs may thus comprise 42 individual LEDs in total.
The LED lighting device comprises a connecting means for connecting the LED lighting device to the AC voltage source. On the one hand, the connecting means provides for the connection to the voltage source, for example by means of a plug-in connector. Furthermore, the connecting means comprises a rectifier, such as a bridge rectifier or diode rectifier. The rectifier is able to modify the AC voltage in known manner so that no negative voltage amplitudes will be applied to the components of the LED lighting device any more, but all voltage amplitudes will extend in the positive direction. Thus, a higher power proportion of the voltage source can be exploited. Moreover, LEDs typically have a pass direction and a reverse direction, and LEDs operated in the reverse direction could possibly even be damaged.
The LED lighting device further comprises a set activation means. The set activation means enables or disables the sets of LEDs, in particular by means of a preset value, more generally by means of an activation instruction. For example, an amperage set point instruction may be provided. In other words, the set activation means defines an amperage target value for each set of LEDs, preferably the same amperage target value for each set of LEDs.
Set activation will be explained in more detail by way of the following example illustrating one way of implementation of the set activation means. In this example, the set activation means comprises a respective current controller assigned to each set of LEDs. Each current controller receives an amperage set point instruction. With the rising edge of the applied voltage source, the first set of LEDs is then supplied with electric power, the LEDs begin to light up. As soon as the voltage is sufficient to connect another set of LEDs in series, current begins to flow through this set of LEDs. In particular, the current controller assigned to the first set of LEDs opens or closes an electrical connection so that then current also passes through the second set of LEDs. In the same way, further sets of LEDs are enabled with further increasing voltage level of the voltage source.
In principle, therefore, the sets of LEDs may be connected and disconnected in response to the measured voltage amplitude even during the time period of an oscillation period (phase) of the AC voltage. Power consumption of the LED lighting device is thus adapted to the power provided by the power supply, even and especially within one oscillation period.
Thus, in a simple example, starting at 0 volts, initially the first set of LEDs is enabled during the rising edge and current can flow through the first set of LEDs. The remaining sets of LEDs thus initially remain disabled, until a threshold defined by the set activation means is exceeded. As soon as the threshold is exceeded, the next set of LEDs is enabled (or disabled in case of a falling edge). In the present example, the second set of LEDs will be enabled once the threshold value has been exceeded. If further sets of LEDs are provided, the third set of LEDs is enabled for instance as soon as a second threshold has been exceeded.
During the falling edge of the voltage waveform, the sets of LEDs may for instance be disabled in the reverse order.
By successively activating and deactivating sets of LEDs during an oscillation period of the voltage source, a power factor corrected (PFC) pseudo-resistive load is thus provided, which can be directly connected to the voltage source.
The desired amperage passing through each set of LEDs may preferably be adjusted by the current controller. In case of a fluctuating voltage source, this allows to adapt the LEDs and in particular the brightness of the LEDs to the applied voltage level. For example, in case of a higher voltage the voltage source exhibits a steeper slope of the phase, so that the sets of LEDs are enabled slightly earlier. In response to the slope of the phase rise, the set activation means may therefore preferably adjust the threshold(s) for the current controller. In case of a steeper phase slope this means that a lower threshold is set, in order to obtain the same light output from the LEDs and thus the same overall brightness as a result.
The threshold for a specific set of LEDs in particular results from the voltage applied. Each connected set of LEDs has associated therewith a current value which can be controlled.
The current controller does not only permit to keep the brightness of the LED lighting device as constant as possible in case of a fluctuating voltage source. The current controller may furthermore be configured so that the brightness of the LED lighting device can be adjusted to a desired value. This may for instance be achieved by adjusting the activation instruction.
With the current control for controlling the current passing through the at least first set of LEDs, brightness fluctuations of the LEDs due to fluctuations of the voltage amplitude of the voltage source can be compensated for. In general terms, the current controller controls the current flowing through the set or sets of LEDs. Preferably, the current controller may control the current for each set of LEDs individually, or a respective separate current controller is preferably provided for each set of LEDs. By employing graduated current control associated with the sets of LEDs, an adjustable good PFC can be achieved.
In a simple case, the current controller may for instance be implemented by operational amplifiers, Darlington transistors, or by FETs, which compare a set point amperage with an actual amperage in the electrical circuit.
Furthermore, by controlling the amperage of the LEDs in a set of LEDs by the current controller, the light output of the LED is adjusted.
The activation instruction may as well comprise a preset voltage value, as will be explained by way of the following example. With the voltage of the voltage source increasing, initially the first set of LEDs is enabled. With the voltage further increasing beyond the threshold voltage, which is for instance just below the maximum voltage tolerable by the individual LEDs, the second set of LEDs can be activated in response to the measured voltage, e.g. by an activation instruction such as a switching signal from the signal generator. The second set of LEDs is preferably electrically connected in series to the first set of LEDs. The voltage drop across each individual LED is thus shifted downstream and the LEDs of both the first and the second set of LEDs will emit light of the same intensity. If the voltage of the voltage source or the voltage drop across each individual LED rises again beyond the threshold voltage, the third set of LEDs is optionally enabled, whereby the voltage drop across each individual LED will be reduced again.
Thus, the individual LEDs can always be operated in a safe range, without causing damage to individual LEDs or to the LED lighting device.
For sensing the phase rise or detecting the voltage amplitude, the set activation means may comprise a voltmeter for measuring a currently applied voltage amplitude of the AC voltage source. The voltmeter detects the voltage amplitude, preferably continuously, for adjusting the threshold value(s) based thereon. The voltmeter preferably integrates over several periods in order to provide for a more uniform light output.
The enabling and disabling of LED sets during an oscillation period causes flickering of the LED lighting device. However, already at frequencies of typical household AC voltages of e.g. 60 Hz (as in the United States of America) or 50 Hz (as in Europe) this flickering is not or not readily noticeable by the human eye. So, all in all, the flickering is tolerable because it does not impair the aesthetic requirements on the LED lighting device. Rather, brightness of the LED lighting device will be perceived as constant. The total luminance emitted by the individual LEDs during one phase is perceived by the human eye as an integrated total value. The contributions of the LED sets to the total luminance will typically be different, since one set of LEDs may have a duty cycle different from that of another set of LEDs, for example. Moreover, frequencies used for power supply in aviation are typically about 400 Hz. Therefore, the flickering is far less remarkable in this case.
In one embodiment, the set activation means of the LED lighting device may comprise a signal generator for generating a switching signal in response to the currently applied voltage amplitude. It is also possible for the sets of LEDs to be switched, i.e. enabled or disabled, by the switching signal. In other words, the activation instruction is implemented by the switching signal in this case.
According to the invention, the individual LEDs are adjacently surrounded by LEDs which are assigned to the respective other set of LEDs. In other words, any LED of the first set of LEDs is arranged so that all adjacent LEDs are not assigned to the first set of LEDs.
Particularly preferably, the LED lighting device is an expanded LED lighting device. In other words, the LED lighting device extends across an area or along a line. It has been found in the context of the invention that light distribution of the expanded LED lighting device is perceived as particularly homogeneous across the area or along the line, when the LEDs of an LED set are arranged so as to be distributed over the area or along the line. The “mixing” of individual LEDs of different sets is very effective when the LEDs of a specific set are not used side by side. A most homogeneous light distribution across the expanded LED lighting device is achieved when LEDs of all employed sets or as many of the employed sets as possible alternate each other.
Preferably, the LED lighting device further comprises a switching means for connecting or disconnecting at least one set of LEDs in response to the switching signal. Thus, the LEDs can be enabled or disabled in sets by the switching means.
The LEDs of the LED sets are preferably uniformly distributed over the area or along the line occupied by the LED lighting device. In a particular embodiment, the LEDs of the LED sets are distributed over the area or along the line occupied by the LED lighting device in a checkerboard-like pattern. Similarly to a checkerboard, the LEDs of the first set occupy a first color, the LEDs of the second set a second color. In the case of more than two sets, the checkerboard analogy may be extended mentally to more than two colors. This could also be called checkerboard-like.
In other words, the LEDs of an LED set are spaced apart from one another. Preferably, the LEDs of a set of LEDs are arranged in distributed manner so that the LEDs of a set of LEDs are distributed over the entire extension of the LED lighting device.
The LED lighting device preferably comprises at least one further LED set, and each LED is adjacently surrounded by LEDs that are assigned to another set of LEDs.
The optional switching means of the LED lighting device may be configured so as to adjust the number of enabled LED sets such that the LEDs of the LED sets are operated near their nominal voltage. When the LEDs are operated at about their nominal voltage, the highest possible light output is resulting, so that the efficiency of the LED lighting device rises. Operation of the LEDs near their nominal voltage may also be achieved by means of the set point instruction for instance for the amperage threshold value.
Preferably, the LED lighting device comprises at least one overvoltage diverting component, or, more generally, a voltage diverting component. The voltage diverting component is preferably capable of absorbing an excess voltage or overvoltage so that the LEDs of the LED sets will not be damaged even if the maximum total voltage across all individual LEDs is exceeded. An overvoltage exists when the voltage amplitude of the supply voltage is above the voltage level for which the LED lighting device is designed when in normal operation.
Rather, the excess voltage is diverted to the voltage diverting component. Most preferably, the at least one voltage diverting component is a non-luminous component, e.g. non-luminous diodes or resistors. Optionally, the voltage diverting component may be defined by a set of LEDs otherwise identical to the other sets of LEDs, but covered or attached so as to be obscured, so that this set of LEDs does not contribute to the overall brightness of the LED lighting device.
Thus, if an overvoltage arises, which may for instance occur regularly in the field of aviation, the LED illumination device will not become more bright or be subject to a brightness fluctuation, in contrast to conventional lamps, rather the additional power from the overvoltage can be diverted by the voltage diverting component.
Preferably, the set activation means is configured for diverting the excess voltage via the at least one voltage diverting component. For this purpose, the set activation means may connect the non-luminous component in series with the sets of LEDs, when needed.
In other words, the non-luminous component is capable of reliably diverting an overvoltage so as to preserve the LED lighting device from damage, while at the same time avoiding undesirable fluctuations in brightness of the LED lighting device.
The LED lighting device may comprise the LEDs of the sets of LEDs mentioned above, which may be assigned to a first diode string, and may further comprise a second diode string with further LEDs, which is electrically arranged in parallel to the first diode string and also comprises further sets of LEDs.
The invention will now be explained in more detail by way of exemplary embodiments and with reference to the figures, in which the same and similar elements are partly designated with the same reference numerals, and wherein the features of the different exemplary embodiments can be combined with each other.